The present invention relates to an internal combustion engine that has an exhaust-driven turbocharger and improved acceleration characteristics.
Preferred objectives during the development of improved drive units for commercial operation are a good combustion start, low consumption, as well as as low an environmental pollution due to engine exhaust as possible. To realize these objectives, these days supercharged engines having a high power density are increasingly used. The boosting of the mean effective pressure is generally drawn from exhaust-driven turbochargers. The additional cooling of the combustion air effects a further increase of the effective mean pressure.
Due to their high specific performance, Diesel engines having exhaust-driven turbochargers and combustion air cooling have a number of advantages over naturally aspirated engines:
the relative proportion of frictional losses at the indicated performance is less, thereby reducing the consumption of fuel; PA1 the smaller overall size permits a more flexible design of the passenger space; and PA1 a lower price due to a lower expense for material.
Opposing the aforementioned advantages is the known problem of the retarded response time of exhaust-driven turbocharged engines ("turbo lag"). In addition, the lack of sufficient air during the acceleration processes leads to a pronounced burst of smoke.
Various proposals have been made to improve the acceleration characteristic. These proposals are essentially based upon the idea of supplying to the engine the missing quantity of air that exists at an increased load state from the outside (air injection), or to provide this air at an earlier point in time via a more rapid operation of the turbocharger.
For example, it is known to realize the injection of the auxiliary combustion air into the intake pipe or manifold by using an auxiliary compressor having an auxiliary drive. In this connection, it would also be possible to use a compressed air tank, although only for driving the auxiliary drive. An appropriate check flap is in this connection disposed in the intake line of the main compressor (see German Patent 31 00 732).
A drawback of this known arrangement is that during the air injection, the entire space of the conduits (up to the then closed check flap) must first be filled, as a result of which the turbocharger is even slowed down for a short period of time (reduction of the speed of the exhaust-driven turbocharger). A particular disadvantage is such an arrangement of the check flap when using a combustion air cooler, which is disposed between the compressor and the intake manifold. Since the auxiliary air is produced by an additional turbocharger, pressure is built up relatively slowly in the system, which readily meets the expectations of an improvement of acceleration for the there existing application, namely use for large engines, primarily at a steady operation. The air injection is in such a case effected relatively seldom, whereby a large quantity of compressed air is slowly generated and is kept available for a longer period.
A more effective air injection, also of the aforementioned general type, is known from U.S. Pat. No. 3,673,796, Weick et al, since here the check flap is disposed in the immediate vicinity of the intake manifold after the compressor. The subject matter of this reference (the speed of the drive motor of a power generator is to be kept constant) similarly relates to an improvement of the load assumption or response characteristic of engines having exhaust-driven turbochargers by providing an adequate quantity of compressed air.
During intermittent vehicle operation, especially of commercial vehicles, where air injections are frequently required, it is desirable, however, to not only improve the acceleration time, but also to eliminate the burst of smoke that occurs during the acceleration (it is particularly desired to achieve a reduction of particulate emission). Heretofore known references offer no solutions for this problem.
In conformity with the desire of the driver, during a normal acceleration process from low load and speed, by stepping on the gas pedal the quantity of fuel to the combustion chamber of the engine is immediately increased, resulting in initiation of acceleration. At this moment, inadequate mixture formation conditions exist in the combustion chamber, i.e. the swirl generated in the intake port, the turbulence in the combustion chamber, as well as the atomization and distribution of the fuel, are too unfavorable for a clean combustion of the increased quantity of fuel.
This known fact applies not only to naturally aspirated engines but also to supercharged engines. Since with supercharged engines, especially turbocharged engines, this problem is greater than with naturally aspirated engines due to the time lag of the charging arrangement, engines with exhaust-driven turbochargers have at their disposal a fuel quantity limitation that is a function of charging pressure (LDA). This LDA regulation ensures that during the acceleration process, the fuel quantity is released in conformity with the charging pressure, but with a necessary acceleration surplus.
The fuel quantity cannot be limited by the LDA in such a way that the exhaust gas quality has the same favorable values as in a steady operation, since otherwise the acceleration of the vehicle would not be acceptable.
As already indicated, even the use of the aforementioned air injection pursuant to U.S. Pat. No. 3,673,796 does not aid in improving the exhaust gas quality, since with this patent, which has the object of keeping a certain speed constant, it is inherently necessary to release the increased quantity of fuel injection into the combustion chamber practically simultaneously with the injection of air into the intake manifold. The result is that in the time between the suddenly increased fuel injection and the time that the compressed air arrives in the combustion chamber, the previously described phase of a lack of sufficient combustion air exists. Under these conditions, a pronounced burst of smoke must be reckoned with.
It is therefore an object of the present invention for internal combustion engines that have an exhaust-driven turbocharger, especially such engines that are used in commercial vehicles that predominantly operate in an intermittent manner, to alter the conventional acceleration sequence, while using an auxiliary combustion air supply of the aforementioned general type, in such a way that a burst of exhaust gas smoke (especially the increase of particulates) is entirely prevented.